Threat Level: green Handler on Duty: Didier Stevens

SANS ISC: Maldoc: Once More It's XOR - SANS Internet Storm Center SANS ISC InfoSec Forums

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Maldoc: Once More It's XOR

I was asked for help with malicious Word document MD5 7ea8e50ce884dab89a13803ccebea26e.

Like always, I first run on a sample:

As expected, it contains VBA macros. Then I quickly look at the source code of the VBA code in all macro streams (options -s a -v):

I noticed a string that looks like BASE64 at the end of the VBA source code (that's why I used a tail command in this screenshot). Checking with my tool confirms that this is indeed BASE64:

The output confirms that it is BASE64, although I don't recognize the binary data (most bytes are not printable characters).

The string is BASE64, and function gFpVdtRecxaZD is most likely a BASE64 decoder function. The return value of this function is used as first argument to function MOMCqdxBOimtoI. Function MOMCqdxBOimtoI takes 2 arguments, the second argument is a printable string.

I've seen this often before, MOMCqdxBOimtoI is most likely a decoding function, and the second string is the decoding key.

What encoding function? First I try XOR encoding, because it's popular. With my tool I check what the result is of XORing the decoded BASE64 string with the key:

I get a readable, known string: MSXML2.XMLHTTP. This confirms that the encoding is indeed XOR and that the second argument is the key.

Grepping for string MOMCqdxBOimtoI shows me all the lines with encoded strings:

I check the longest string first, because that's most likely the URL:

This analysis can also be automated with plugins.

My oledump plugin plugin_http_heuristics was not able to decode the URL of this sample, until I made a small change:

I'll explain the changes to this plugin in the next diary entry.


Didier Stevens
Senior handler
Microsoft MVP


650 Posts
ISC Handler
Oct 13th 2018
Do current versions of Word disable support for .DOC by default yet? It would seem to be a good idea. But they also open it in a kind of sandbox, IIRC. So what versions of Word are vulnerable to this and are they in default configuration?
Larry Seltzer

26 Posts
And it would seem that WebSense/Triton hasn't seen it either. It's "uncategorized" as of now as well.

23 Posts
No, .doc is fully supported.

650 Posts
ISC Handler

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